I found a really nice podcast interview with John Traxler, who is very much a pioneer in the area of mobile learning and research.
The bit that interested me was when he talked about how the model that institutions (universities etc) deploy for elearning is just not suitable for mobile learning and students will not respond well to being issued a standardised mobile device when they probably already have one that they like.
Certainly most of the research studies I've read into the subject involve institutions buying a large quantity of identical mobile devices (PDAs or iphones, ipod touches etc) and then issuing them to the students. As Traxler said, this is very much based on the model they use for desktop computers where they buy a load of them and put them in a computer lab for student use.
This is problematic because it's likely that student already have a device they use and now they either have to abandon their first device or carry around two, which will turn out to be impractical for a lot of people. I know that I would get annoyed having to always remember to take both devices with me and the convenience of mobile learning would suddenly become an encumbrance.
Also - again something Traxler points out - users tend to be quite possessive of their mobile devices and have preferences that may not match up with the universities. They might not like certain form factors (e.g. touchscreens or flip phones) or even certain brands (go to some mobile phone forums if you want to see some wonderful examples of fanboy pettiness) and would prefer to either have a choice over which phone they can have or just to keep the one they've already got.
I think institutions are going to have to be a lot more flexible when it comes to mobile learning than elearning. They are going to have to be ready to give up some control so that students feel comfortable with it. For example, they will not be able to control exactly what they do with them as they can do at the moment with desktop computers. And they are going to have to offer different avenues for having content delivered to them: eg. not just via mobile internet, but SMS or bluetooth transfer to account for the wide range of mobile devices that students have.